The present invention relates to an exposure apparatus, an exposure method, and an exposure system.
A conventional reduction projection exposure apparatus projects a circuit pattern of a reticle (mask) onto a wafer or another substrate via a projection optical system in manufacturing fine devices, such as a semiconductor memory and a logic circuit, using the photolithography technology.
The minimum critical dimension (“CD”) (or a resolution) transferable by the reduction projection exposure apparatus is proportionate to a wavelength of the light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is and the higher the NA is, the smaller the resolution is. Along with the recent demand for the fine processing to a semiconductor device, use of a shorter wavelength of the exposure light is promoted. For example, use of the ultraviolet (“UV”) light having a shorter wavelength is promoted from a KrF excimer laser (with a wavelength of approximately 248 nm) to an ArF excimer laser (with a wavelength of approximately 193 nm).
With this background, the immersion exposure is one attractive resolution improving technology that uses a light source, such as the ArF excimer laser. The immersion exposure increases an apparent NA of the projection optical system and improves the resolution by filling the liquid in a space between the final lens of the projection optical system and the wafer (or by replacing the medium at the wafer side of the projection optical system with the liquid) and by shortening the effective wavelength of the exposure light. The NA of the projection optical system is defined as NA=n×sin θ, where n is a refractive index of the medium. The NA increases up to n when the medium has a refractive index higher than the air's refractive index, i.e., n>1.
Since liquid having a higher refractive index is expected to provide a higher resolution, an immersion exposure apparatus that uses liquid having a high refractive index (“high refractive index (“HRI”) liquid”) is proposed as a successor technology of an immersion exposure apparatus. See Japanese Patent Application No. 2006-4964.
However, as a result of a comparison between pure water and the HRI liquid, this inventor has discovered that the immersion exposure apparatus that uses the HRI liquid poses the following problems different from the immersion exposure apparatus that uses the pure water:
The first problem about the immersion exposure apparatus that uses the HRI liquid is a necessity of a recycle of the liquid because the HRI liquid is more expensive than pure water and more influential to the environment.
Some immersion exposure apparatuses that circulate the liquid have already been proposed. See U.S. Pat. No. 4,346,164 and Japanese Patent Application, Publication No. 2005-136374.
Each of the immersion exposure apparatuses disclosed in these references removes impurities from the recovered liquid (or purifies it by the purifier), and recycles the liquid. In general, a detector that detects the purity of the purified liquid sits subsequent to the purifier. For example, Japanese Patent Application, Publication No. 2005-136374 proposes a method for detecting a particle amount and an impurity amount, and a method for measuring a physical characteristic, such as an electric resistance and a refractive index. However, studies by this inventor have revealed that this detector cannot precisely detect the purity of the liquid. In other words, the conventional immersion exposure apparatus cannot well guarantee that the circulated or recycled liquid has good quality, and may recycle liquid that is too inferior to recycle. This is a peculiar problem to the immersion exposure apparatus that uses the HRI liquid to circulate the high quality liquid, and is irrelevant to the immersion exposure apparatus that uses pure water.
The second problem that uses the HRI liquid is that the HRI liquid's transmittance is likely to degrade. Oxygen is more likely to dissolve in the HRI liquid than in the pure water, and thus the HRI liquid remarkably reduces the transmittance to the UV wavelength range when exposed to the air. Although the conventional immersion exposure apparatus includes a degassing unit that reduces all dissolved gases to restrain gas bubbles, the degassing unit cannot well eliminate dissolved oxygen, and a small amount of dissolved oxygen residue reduces the liquid's transmittance. In addition, it is feared that a reaction, such as decomposition due to the exposure light causes a drop of the transmittance. As the liquid's transmittance reduces, its temperature rises due to the absorption of the exposure light and its refractive index changes, causing the exposure aberration. Strict control over the liquid's transmittance is required to secure the exposure or imaging characteristic. In correcting an exposure aberration caused by the liquid temperature change, it is preferable to maintain the liquid's transmittance constant. When the liquid's fluctuates, an amount of aberrational correction needs to be controlled in accordance with the fluctuation.